The present invention relates to a single crystal growth apparatus for pulling up a single crystal while a magnetic field is applied to a single crystal material melt.
When a single crystal is grown in accordance with Czochralski growth, a single crystal material melt 2 stored in a crucible 12 is heated by a heater 14 so as not to permit the melt to soldify, as shown in FIG. 1. In this state, a seed crystal 8 is dipped in the melt 2. When the seed crystal is pulled by a seed shaft lift and rotation mechanism 10 at a predetermined speed, a crystal is grown in a solid-liquid interface layer 4, thereby preparing a single crystal 6.
The melt 2 is convected in the direction indicated by each arrow 16 upon energization of the heater 14. This thermal convection occurs when a buoyancy, by thermal expansion of a fluid, is unbalanced with a viscosity thereof. A dimensionless number representing the balance between the buoyancy and viscosity is Grashof number EQU N.sub.GR =g.multidot..alpha..multidot..DELTA.T R.sup.3 /.nu..sup.3 ( 1)
where
g: gravitational acceleration, PA1 .alpha.: the thermal expansion coefficient of the material melt, PA1 .DELTA.T: the temperature difference along the radial direction of the crucible, PA1 R: the radius of the crucible, and PA1 .nu.: the dynamic viscosity coefficient of the material melt. PA1 .mu.: the magnetic permeability of the melt, PA1 H: the magnetic field intensity, PA1 D: the crucible diameter,
When the Grashof number N.sub.GR exceeds a critical number determined by the geometric dimensions and the thermal boundary condition of the melt, thermal convection occurs in the melt. In general, when N.sub.GR exceeds 10.sup.5, the thermal convection of the melt results in a turbulent flow. Furthermore, when N.sub.GR exceeds 10.sup.9, the thermal convection results in turmoil. In normal operation, a single crystal ingot having a diameter of 3 to 4 inches is prepared by pulling up a seed crystal. However, in this case, the melt is in a state of turmoil. For this reason, the material melt surface and the solid-liquid interface layer 4 is waved.
When thermal convection causes a turmoil state, a temperature change at the solid-liquid interface in the material melt is increased. The thickness of the layer 4 changes locally or over time, and, for this reason, the crystal is remelted microscopically during crystal growth, with dislocation loops and bulk stacking defects being formed in the crystal. These defects occur non-uniformly along the single crystal pulling direction since the solid-liquid interface changes irregularly. The melt 2, at a temperature of, for example, 1,500.degree. C., is in contact with the inner surface of the crucible 12 so that a getter impurity is dissolved from the crucible 12 into the melt 2. The mobile impurity is dispersed by thermal convection (indicated by arrow 16) throughout the melt 2. Since the impurity is gettered or captured in the bulk stacking defects to cause dislocation loops, various types of defects and growth fringes are formed in the single crystal ingot or boule, thereby degrading the quality of the single crystal.
When the single crystal ingot is sliced to prepare large scale integrated circuit (LSI) wafers, a wafer including the defects cannot be used and is, subsequently, disposed of since its electrical characteristics are degraded. For this reason, the yield of wafers is low. Although demand has arisen for an increase in the diameter of a singel crystal ingot, when the crucible radius is increased the Grashof number N.sub.GR is also increased, as is apparent from equation (1), and, as a result, thermal convection of the melt 2 becomes more significant such that the quality of the resultant single crystal ingot is degraded.
In a single crystal growth apparatus, as shown in FIG. 2A, a DC magnetic field is applied by a coil 18 to the melt 2 so as to pull up the single crystal under growth conditions of thermal and chemical equilibrium by suppressing thermal convection. A uniform magnetic field is applied by the coil 18 to the melt 2 in a direction indicated by arrow 20, the melt being electrically conductive. When the melt 2 is moved by thermal convection in a direction which is not parallel to the direction of the magnetic field, the melt 2 receives a reluctance in accordance with Lenz's law. Thermal convection of the melt 2 is thus suppressed. In general, a magnetic viscosicity coefficient .nu..sub.E representing the reluctance upon application of a magnetic field is expressed by equation (2) below: EQU .nu..sub.E =(.mu.HD).sup.2 .sigma./.rho. (2)
where
.sigma.: the electrical conductivity of the melt, and
.rho.: the density of the melt.
As is apparent from equation (2), when the magnetic field intensity is increased, the magnetic viscosity coefficient .nu..sub.E is also increased, but, when the dynamic viscosity coefficient .nu. is increased, the Grashof number N.sub.GR is greatly decreased. When a magnetic field intensity exceeds a specific value, the Grashof number N.sub.GR decreases below the given critical value. When a magnetic field having an intensity exceeding the specific value is applied to the melt 2, thermal convection of the melt 2 is almost stopped. As a result, the impurity is not gettered in the stacking defects in the single crystal ingot. The dislocation loop, other defects and growth fringes will not, consequently, be formed. The quality of the single crystal ingot is uniform along the pull-up direction, thereby increasing the yield of the ingots.
However, in a single crystal growth apparatus for manufacturing single crystal ingots, each having a diameter of 4 inches or more, the crucible 12 has a diameter of 6 inches or more, making it very large. In addition, a chamber 21 for housing the crucible 12 and a heater 14 has an inner diameter of several hundreds of millimeters. As a result, the single crystal growth apparatus has, as a whole, a very large size.
In general, the diameter of the crucible 12 is larger than the depth thereof. When a maximum amount of melt 2 is charged in the crucible 12, the radius of the crucible is substantially the same as the depth thereof. When a magnetic field is applied to the melt in the crucible 12, a magnetic field intensity distribution is as shown in FIG. 2B. The temperature distribution of the melt 2 is uniform along the direction of the height of the crucible 12 due to the diameter/height ratio. A relationship between a magnetic field intensity B1 in the layer 4 and a magnetic field intensity B2 at the lower portion of the crucible 12 is given by the following inequality: EQU .vertline.(B1-B2)/B1.vertline.&lt;5%
where symbol .vertline. .vertline. represent an absolute value. A Grashof number distribution of the melt 2, corresponding to the magnetic field distribution shown in FIG. 2B, is less then the critical value N.sub.GC for the melt 2 throughout the crucible 12, as shown in FIG. 2C. Referring to FIG. 2C, N.sub.G1 and N.sub.G2 correspond to the Grashof numbers of a melt portion near the layer 4, and a melt portion near the bottom of the crucible 12, respectively. For this reason, in the conventional crystal growth apparatus, thermal convection is stopped throughout the melt 2 in the crucible 12, and the melt 2 is completely motionless. In this state, heat conduction by convection will not occur. Heat is conducted from the heater 14 to the melt 2 at the center of the crucible 2.
When a single crystal ingot diameter is as small as 2 to 3 inches, an inner diameter of the crucible 12 can be as small as 4 to 5 inches. For this reason, even if the melt is completely still upon application of the magnetic field, heat from the heater 14 is sufficiently conducted to the layer 4. For this reason, a temperature difference between a melt portion near the layer 4 and a melt portion near the crucible 12 normally falls within a range between 10.degree. and 20.degree. C. However, when a single crystal ingot diameter is increased to 4 inches or more, the diameter of the crucible 12 needs to be 6 to 14 inches, greatly increasing the crucible size. Consequently, heat from the heater 14 cannot be sufficiently transferred to the layer 4 by thermal conduction in the melt 2 alone. For this reason, a large temperature gradient of several tens of degrees centigrade is generated between the melt portion of the layer 4 and the melt portion near the inner surface of the crucible 12. In order to effectively grow a single crystal 6 in the layer 4, a temperature of the layer 4 must be sufficiently higher than that of the melt. In order to manufacture a large-diameter single crystal ingot, the power of the heater 14 is increased to compensate for a decrease in temperature due to the temperature gradient, and to set the layer 4 at a predetermined temperature. When the single crystal ingot diameter is increased, and a temperature gradient in the melt is large, a temperature gradient occurs in the layer 4. The temperature gradient in the layer 4 prevents uniform single crystal growth. When a temperature difference between the center of the crucible and the melt portion near the inner surface of the crucible is excessively large, an excessive thermal stress acts on the crucible 12 such that the crucible 12 tends to crack.
When a large-diameter single crystal ingot is manufactured by a conventional single crystal growth apparatus, the temperature gradient at the center of the melt and that at the solid-liquid interface layer is increased excessively, resulting in inconvenience. Therefore, a uniform single crystal ingot cannot be obtained, high power is required to heat the melt, and the crucible tends to crack.